In data handling systems and in particular in computer systems, the concept of enlarging the capability and/or capacity of a basic system into a medium-sized system or even a large-scale system, by adding modular units to the basic system, has long been pursued. However, heretofore a system which permitted such "modular" addition and removal required that the system provide a number of hardware (circuitry) features to accommodate the addition and removal of the modules (i.e., interface circuits and peripherals). For instance, with respect to a first feature of prior art systems, each interface circuit card has resident decoding circuitry as part thereof, so that when the master system wants to communicate with a modular device, the interface circuit can be addressed, irrespective of its physical location amongst the slot means which hold such interface circuit (cards). Such decoder circuitry has been fabricated, in the prior art, such that for each modular device, of a given type (e.g., a floppy disc device), a standard address circuitry configuration is provided, and if the standard address were to necessitate a change, jumpers or wirewrap changes are added or made to the address decoding circuitry to "change the address." Thirdly, while some prior art systems do permit a certain amount of interchangeability (because each interface circuit is addressable), such systems have normally required that the interface circuit cards be particularly placed in an ordered arrangement of slots to implement a method for determining priority amongst a group of active interface circuits.
The circuitry which effects a serial scanning of the "ordered" slots has, in the prior art, been some form of "daisy chain" configuration and such configurations require that there be no empty or vacant slots between active interface circuit cards. This constraint is suffered by prior art circuits. In addition, as the number of possible modular peripherals, which could be added, were in fact increased, the amount of memory space used for I/O devices, addresses, per se, increased. Further since the diagnostic routine instructions, in the prior art, use memory space in the main system, it follows that as the number of modular devices, which optionally could be added, were in fact increased, the amount of memory space used by diagnostic routine was increased. The increased use of memory space, described above, of course reduces the amount of memory space available for use with problem solving programs. In the prior art such memory restraints are overcome by adding memory capacity at additional costs.
The present invention: eliminates the necessity of having address decoding circuitry on each interface circuit card; eliminates the "ordering" of interface circuit cards in particular slots, to accommodate a priority determination plan; permits vacant slots to exist between active interface circuit cards; eliminates excessive use of memory space to accommodate addresses for increased numbers of optional peripheral devices; and eliminates excessive use of memory space to accommodate additional diagnostic instructions as the number of optional peripheral evices is increased. It is to the foregoing concerns that the present invention offers a solution.